Reflective displays reflect ambient light similar to printed images; by contrast, conventional flat panel displays require a separate light source called a backlight. The brighter the ambient light the brighter a reflective display becomes, and this is particularly noticeable in sunlight and outdoor applications as the screen will be just as comfortable to view as it is indoors, however a backlit screen that appears bright indoors will become washed out and difficult to read outdoors. For example, the screen of a backlit mobile/cell phone can be difficult to read in direct sunlight. Applications that favor reflective displays include ebook readers, mobile devices for outdoor and indoor use, outdoor billboard displays, public information displays, see-through window displays, digital advertising displays, signage, and displays in automotive applications.
Reflective display technologies include reflective twisted nematic liquid crystal, reflective guest host liquid crystal, chiral nematic liquid crystal, polymer dispersed liquid crystal, electrophoretic ink, electrowetting, quick response liquid powder, interferometric modulator display/MEMS, and electrochromic.
Within the prior art reflective display technologies two have been demonstrated with a technique that involves microencapsulation of an electro-optical fluid: liquid crystal and electrophoretic ink. Microencapsulation preserves the fluid nature of an electro-optical fluid inside a solid form—discrete, sealed volumes enclosed inside a polymer wall or skin and referred to as a shell or capsule—before being coated onto a substrate and used in a display device. The known techniques for microencapsulation can be divided into two types:                a) The hydrophilic shell (synonymous with capsule) type: the bulk polymer in a shell has a dominant hydrophilic nature. Generally the prior art methods require the majority by weight of the polymer wall precursors to be water soluble and to be solubilized in an aqueous phase at some point during the microencapsulation process.        b) The hydrophobic shell (synonymous with capsule) type: the bulk polymer in a shell has a dominant hydrophobic nature. Generally the prior art methods require the majority by weight of the polymer wall precursors to be soluble or partly soluble in the electro-optical fluid to be microencapsulated.        
The former type includes conventional and complex coacervation, and interfacial polymerization processes, and up to now coacervation has been demonstrated for electrophoretic ink microencapsulation.
The hydrophilic-shell type of microencapsulation uses at least one significant polymer wall component that is soluble in an aqueous phase and so the resulting shell or capsule wall polymer will be permeable to moisture, or even hydroscopic. Normally, a barrier to the ingress of atmospheric moisture has to be used such as a hermetic sealing wall around the display (i.e., an edge seal). Eink Holdings' U.S. Pat. No. 7,109,968 discloses a technique for microencapsulating electrophoretic ink using a complex coacervation process, and U.S. Pat. No. 6,120,701 discloses a technique for microencapsulating nematic liquid crystal using an interfacial polymerisation process.
In the hydrophobic-shell type, a shell's wall is generally formed from hydrophobic monomers having a slight hydrophilic functionality. Ahead of microencapsulating, the monomers are solubilized partly or completely in the hydrophobic, electro-optical fluid and as polymerization proceeds the polymer phase separates from the electro-optical fluid and forms at the interface with the aqueous phase and microencapsulates the electro-optical fluid with a hydrophobic polymer wall. For examples of liquid crystal microencapsulated in this way, see Rohm and Haas Company's U.S. Pat. No. 5,976,405 or PolyDisplay's U.S. Pat. No. 7,397,530.
Once an electro-optical fluid is microencapsulated it can be dispersed in a solution containing polymer matrix (synonymous with binder) precursors to form an electronic ink. This electronic ink can be coated or printed onto a variety of flexible and rigid substrates. The coated/printed ink is then formed into a solid (by curing or film forming techniques) consisting of discrete volumes of electro-optical fluid enclosed inside polymer shells that in turn are dispersed in a polymer matrix to form the electro-optical layer of a reflective display device. The electro-optical fluid and shells are referred to as the discontinuous phase while the polymer matrix/binder is referred to as the continuous phase, and generally the electro-optical layer is said to comprise two phases.
An important characteristic of the electro-optical layer of such devices is the reflectivity of the white state and how this compares with paper. A viewer's perception of the quality and suitability of a reflective display device may well be influenced by how close its white state is perceived to match the whiteness of paper. In addition, if a display were to have high white-state reflectivity, it could favorably be used with an overlaid color filter to make bright color displays. The color filter comprises a matrix of three or more colors arranged in so called Red-Green-Blue or PenTile (trademark of Samsung) matrices, and works with a monochrome (i.e., black and white) electro-optical layer (i.e., liquid crystal or electrophoretic Ink) to define addressable pixels (picture elements). Each pixel comprises three or more subpixels with the subpixels directing white light at their corresponding color areas in the filter. The eye combines the color and light intensity of the subpixels to resolve a single color and brightness level for a given pixel.
But the perceived whiteness and light reflectance of prior art microencapsulated, electro-optical devices is significantly poorer than white paper, and a side-by-side comparison shows the former to be grey in appearance rather than white. Hence, there is a need for reflective display devices having improved whiteness and light reflectance, and improved reflectance/brightness in color applications.